diff --git a/src/PeriodCollection.php b/src/PeriodCollection.php
index 0412810..94dfbf4 100644
--- a/src/PeriodCollection.php
+++ b/src/PeriodCollection.php
@@ -205,7 +205,7 @@ public function union(): PeriodCollection
     {
         $boundaries = $this->boundaries();
 
-        if (!$boundaries) {
+        if (! $boundaries) {
             return static::make();
         }